Document Custom WordPress builds

[adamziel]

Playground now uses .zip files instead of .data files for loading WordPress. It still requires the SQLite plugin to be pre-loaded and there is no documentation – let's make sure there is a documentation section that explains how to create a custom WordPress build.

Previous description

This is no longer up-to data:

As more developers produce custom wordpress.data bundles to run in Playground, it makes sense to provide an easy way of doing that.

The current process requires diving into the repo, finding the WordPress-bundling Dockerfile, adjusting it, finding the produced .data bundle, and figuring out a way to use it in your project.

Ideally, there would be a command like bundle-wordpress ./my-wp --out=my-bundle.data that would do it in a single step. Ideally, it would be a JavaScript/wasm npm package with no dependency on docker and no Dockerfiles involved. Brownie points if we could have an Export to .data button on playground.wordpress.net to create such a bundle without writing any code at all.